The present invention relates to a method of measuring sizes in scanning microscopes, in particular for precision measurements of small sizes.
More particularly it relates to precision measurements of small sizes with the use of scanning measuring systems and can be used in high technology processes, in particular in microelectronics for measurement of photo resistive masking features during manufacture of integrated circuit (IC).
Accurate measurements for example of trapezoidal shaped elements with sizes within a submicron and nanometer ranges in scanning electron microscopes have a general scientific and a significant applied importance, since typical objects for such measurements in a micro electronic technology are trapezoidal shaped elements: strips (lines) or rectangles (contact holes) which have a cross-section with a trapezoidal shape and are formed in a layer of photo resist. For many reasons in these cases it is not recommended to use scanning of objects of measurement with a beam of electrons with high energy (more than 5 KeV). Careful scanning with electron beams of low energy (200-1500 eV) leads to specific shapes of video signal, with which it is problematic to determine an accurate value of the size with the use of known algorithms. It is believed that this problem has to be resolved.
Dynamics of developing of microelectronic technology is such that in accordance with the ITRS roadmap in 3-4 years the industry will produce integrated circuits with project rules 100 nm and less. The accuracy of measurements of sizes of the elements of such microcircuits during the process of their manufacturing (interoperational measurements) must be exceptionally high, and an acceptable error corresponds to the value 1.0-1.5 nanometers (3 sigma). At the present time there are no technical solutions which can satisfy these requirements. The problem of the measurements is not to find a method of measurements of such small sizes itself, but to provide a required accuracy of measurements. In view of the economical considerations, the majority of the measurements is performed at the stage of forming of relief photoresist mask resulting from the photolithographic operation. Because of many reasons, side walls of the photoresist elements can not be made exactly vertical. A typical shape of the cross-section is a trapeze. The sizes of elements of integrated circuits formed by etching through a photoresist mask are determined by position of the lower edge of the trapezoidal cross-section of the photoresist mask. Therefore a typical object of measurement is a trapezoidal shaped photoresist element in form of a ledge or a trench with a trapezoidal cross-section. The most important size of such an object is its size in the lower area of the trapeze.
The most widely used process of measurements with scanning electron microscope with a digital scanning system is shown in FIG. 1. It is assumed that the size L of any object to be measured can be calculated as a multiple of two values, namely a size of this object expressed in pixels (LP) and determined in a process of steps related to a left branch in FIG. 1, and a length of pixel (PL) calculated in accordance with the right branch of the same figure:L=LP·PL
As mentioned above, the main problem which is not resolved is to provide accuracy of measurements. There are four main sources of errors in the measurements:                errors connected with the problem of localization of the edges of the object to be measured on its enlarged scanning electron microscope image or a left branch of the figure;        errors which are introduced as a result of the operation of calibration of magnification of the microscope related to the right branch;        errors which are caused by unavoidable noise of the video signal in the scanning electron microscope related to the both branches of the figure;        errors caused by a strategy of measurements which is shown in FIG. 1 and expressed in the above presented formula.        
In order to explain these errors the following has to be explained first. A size of any object is a distance between its edges. In order to determine the size of the object in accordance with its image produced by the scanning electron microscope it is necessary to localize on the image the points which correspond to the edges of the object and to measure a distance between these points with the taking into account the magnification of the microscope. Incorrect or inaccurate procedure of localization of the edges leads inevitably to measurement errors.
Errors of calibration of the magnification (or a pixel length PL calculated during this operation) is incorporated in the value of the size L. Noises of the video signal lead to scattering of both values in the right side of the above mentioned formula, LP and PL. As a result of the corresponding research it has been determined that due to the incomplete of scan linearity of any real scanning electron microscope, the length of pixel PL in practice is not a constant, but it is changed along the field of view. Therefore, the use of this strategy leads to additional errors which increase with the reduction of the linearity of the scan system of a microscope.
A comparison of the measurements of the same object with different scanning electron microscopes show that differences in values can reach several tens of nanometers, that corresponds to tens of percentages if the object to be measured has the size of about 100 nanometers. This is why the problem of localization of the edge of the element to be measured on its enlarged image is not yet resolved and is considered the major problem in size metrology. The approaches with a threshold crossing and linear approximation which are known in the art includes intuitive assumptions, and therefore such algorithms can provide only evaluating measurements with errors in tens of percentages. This is fully applicable to the problem of localization of the lower edge of a trapezoidal object.